The present invention generally relates to nonvolatile memory devices. More particularly, the present invention relates to a magnetic memory cell construction for use in nonvolatile memory devices.
One type of nonvolatile memory device known in the art relies on magnetic memory cells. Known as magnetic random access memory (MRAM) devices, these devices include an array of magnetic memory cells. The magnetic memory cells may be of different types. For example, a tunneling magnetic junction (TMJ) memory cell or a giant magnetoresistive (GMR) memory cell.
The typical magnetic memory cell includes a layer of magnetic film in which the magnetization is alterable and a layer of magnetic film in which the magnetization is fixed or xe2x80x9cpinnedxe2x80x9d in particular direction. The magnetic film having alterable magnetization may be referred to as a data storage layer and the magnetic film which is pinned may be referred to as a reference layer. The data storage layer and the reference layer are separated by a layer of insulating material.
Conductive traces (commonly referred to as word lines and bit lines, or collectively as write lines) are routed across the array of memory cells. Word lines extend along rows of the memory cells, and bit lines extend along columns of the memory cells. Located at each intersection of a word line and a bit line, each memory cell stores the bit of information as an orientation of a magnetization. Typically, the orientation of magnetization in the data storage layer aligns along an axis of the data storage layer that is commonly referred to as its easy axis. External magnetic fields are applied to flip the orientation of magnetization in the data storage layer along its easy axis to either a parallel or anti-parallel orientation with respect to the orientation of magnetization in the reference layer, depending on the desired logic state.
The orientation of magnetization of each memory cell will assume one of two stable orientations at any given time. These two stable orientations, parallel and anti-parallel, represent logical values of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d. The orientation of magnetization of a selected memory cell may be changed by supplying current to a word line and a bit line intersecting the selected memory cell. The currents create magnetic fields that, when combined, can switch the orientation of magnetization of the selected memory cell from parallel to anti-parallel or vice versa. Additionally, the write lines can be used to read the logic values stored in the memory cell.
The layers of magnetic material in the memory cell are typically formed as geometrically patterned films such as squares or rectangles, although other shapes are known to be used. One disadvantage of patterned magnetic layer storage structures is that multiple magnetic domains may form in the magnetic layers, rendering the state of the memory cell indeterminate during read operations. Specifically, patterned magnetic layers generate a magnetostatic field that tends to demagnetize the layer. At the edge of the patterned film, this demagnetization field is typically larger than any anisotropy terms maintaining the magnetization perpendicular to the pattern edge. As a result, the magnetostatic field tends to rotate the magnetic vector of the film near the pattern edges.
The magnetization rotation near the edges of the patterned layer form domain walls within the magnetic film, creating multiple domains of magnetic vectors where the magnetic vectors of the domains are not all aligned. When reading the magnetic memory elements, the multiple domains tend to create noise or areas of varying resistance across the memory cell that makes determination of the state of the memory cell difficult or impossible. In addition, variation in the domain states can produce fluctuations in the switching field that can render the memory cell writing process unpredictable.
From the above, it can be seen that maintaining the direction of the magnetization field in the magnetic layers is important. In the case of the fixed or pinned magnetization field of the reference layer, it is thus desirable to fix the field in a manner which minimizes the presence of multiple magnetic domains. In one common memory cell construction, the pinned reference layer may have its magnetic field fixed by interfacial exchange coupling with an adjacent antiferromagnetic layer. An example of the use of an antiferromagnetic layer is illustrated in U.S. Pat. No. 5,650,958 to Gallagher et al. The use of antiferromagnetic materials has several disadvantages including the need for annealing the materials in a magnetic field, corrosiveness of typical antiferromagnetic materials, and the added manufacturing complexity introduced by the presence of the antiferromagnetic layer. It is thus desirable to eliminate the use of antiferromagnetic materials in the construction of magnetic memory cells to reduce or eliminate the disadvantages associated with the use of those materials.
A magnetic memory device has a plurality of write lines and a plurality of memory cells. Each of the plurality of memory cells are operatively positioned between a corresponding pair of the plurality of write lines. Each of the plurality of memory cells has a sense layer and a reference layer separated by an insulating layer. In one embodiment according to the invention, the reference layer of each of the plurality of memory cells includes a high coercivity permanent magnet which provides a permanently oriented magnetic field without relying on an antiferromagnetic pinning layer.